The present invention relates to a flame retardant thermoplastic polyurethane resin composition having excellent formability and water resistance.
A thermoplastic polyurethane resin has the advantages of both rubber and plastic materials and is used in a variety of applications such as tubes, electric wire insulation, and sheets. According to a method of molding such a resin, resin pellets are heated, plasticized, and melted, and the molten resin is molded by conventional thermoplastic resin molding equipment such as injection molding, extrusion molding, or the like.
In recent years, strict flame retardancy is required for the electric wires and the like in order to prevent accidents. From this point of view, a halogen flame retardant such as a halogen-containing phosphoric ester flame retardant is often mixed in the thermoplastic polyurethane resin.
A thermoplastic polyurethane resin having low hardness (e.g., Shore hardness A80 complying with test method ASTM 2240-75) is melted at about 180.degree. C. to mold it. As the hardness of the resin is increased, it becomes harder to plasticize it. Thus, a thermoplastic polyurethane resin having high hardness (e.g., Shore hardness D75) is melted at about 240.degree. C.
When molding continues for a long time under such temperature conditions, surface roughing or scorching occurs due to deterioration or thermal decomposition of the polyurethane resin left in the molding machine. As a result, the product often has poor appearance when molding continues. For this reason, the molding machine must be stopped, the interior of the machine must be cleaned and washed, and molding must be started again, thus greatly reducing productivity.
In particular, in extrusion molding, a molded body preferably has a uniform thickness along its longitudinal direction. However, since viscosity changes caused by a chemical reaction such as the above-mentioned thermal decomposition occur in the extruder, the amount of delivery changes and a molded body having a uniform thickness cannot often be obtained.
Continuous long-time formability of the thermoplastic polyurethane resin containing a halogen flame retardant is inferior to that of other thermoplastic resins such as polyethylene or polyvinyl chloride resin for the following reasons:
(i) An urethane bond is thermally weak, and a free NCO group generated at a molding temperature of 180.degree. C. or higher tends to react with the urethane group to form a three-dimensional bond (crosslinking bond) such as an allophanate bond in the molding machine; and
(ii) A halogen compound such as hydrogen halide generated by halogen elimination from the above-mentioned halogen flame retardant accelerates deterioration of the resin.
The water resistance of the resultant molded body tends to be undesirably degraded by hydrolysis caused by the hydrogen halide produced by decomposition of the flame retardant.